Portal hypertension and its complications account for much of the morbidity and mortality associated with cirrhosis. The long-term goal of our lab is to understand the molecular control mechanisms of portal hypertension. Sinusoidal vasoconstriction is an essential component in the development of portal hypertension and occurs through contraction of hepatic stellate cells (HSC). However, the process by which these contractile HSC organize and wrap around the sinusoidal channel in cirrhosis, is enigmatic. Our studies have focused on migration signals in HSC that contribute to this process of sinusoidal remodeling. Our exciting preliminary data demonstrates that 1) the small GTPase Rac and the actin-binding protein, VASP, promote HSC migration by generating finger-like filopodia from the plasma membrane, 2) Ableson tyrosine kinase (c- abl) promotes HSC-mediated formation of vascular tubes in vitro, 3) the multifunctional cytokine, nitric oxide (NO) inhibits filopodia formation and HSC-driven vascular tube formation through a protein kinase G (PKG)- dependent pathway, and 4) defects in migration signaling in HSC from animal models of portal hypertension are associated with increased sinusoidal coverage by HSC and an ensuing increase in portal pressure, pointing to an in vivo relevance of the proposed pathways. We have parlayed these novel findings into the central hypothesis that; Rac, acting in tandem with its regulatory partners, VASP and c-abl, maintains a molecular counterbalance with NO that governs the formation of membrane filopodia and HSC-driven vascular tubes. Alterations in this counterbalance influence portal pressure by regulating the level of coverage of the sinusoids by contractile HSC. The Specific Aims of the proposal are to test the three subhypotheses that: 1) Rac dependent filopodia formation in HSC is inhibited by NO through PKG dependent inactivation of the Rac effector protein, VASP, 2) C-abl governs HSC-driven vascular tube formation through pathways regulated by Rac and NO, and which are augmented in portal hypertension, and 3) the counterbalance of Rac and NO in HSC regulates sinusoidal remodeling and portal hypertension in vivo. Thus, this proposal will use state-of-the- art methodologies and world-renown collaborators, to elucidate novel signaling pathways in HSC that contribute to portal hypertension, which in turn will set a trajectory towards new therapeutic approaches towards portal hypertension and its symptoms.